1. Field of the Invention
The present invention relates to a light emitting diode circuit, and more particularly to a light emitting diode circuit having even current.
2. Description of Related Art
Back light modules are commonly used now and may use cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs) as light emitting sources. However, LEDs have some advantages in being light emitting sources of the back light modules, such as saving power, small size, etc. Furthermore, mercury is required for lighting CCFLs, and the mercury is a toxic in nature. Therefore, many manufacturers produce back light modules using LEDs as light emitting sources.
With reference to FIG. 3, a conventional LED circuit of a back light module comprises multiple LED strings (50) and a current equalizing integrated circuit (IC) (60). The LED strings (50) are connected in parallel, and each LED string (50) comprises multiple LEDs (51) connected in series and has a first end and a second end. The first end of each LED string (50) is connected to an external power source. The current equalizing IC (60) is connected to the external power source and the second ends of the LED strings (50), has a bias pin and multiple driving pins and may comprise a multi-transistor current mirror. The bias pin is connected to the external power source to obtain a bias voltage. The driving pins are connected respectively to the second ends of the LED strings (50). The multi-transistor current mirror may be a multi-bipolar junction transistor (BJT) current mirror comprising a reference transistor (QR) and multiple current transistors (Q). The reference transistor (QR) has a base terminal, a collector terminal and an emitter terminal. The collector terminal of the reference transistor (QR) is connected to the bias pin and the base terminal of the reference transistor (QR). The emitter terminal of the reference transistor (QR) is connected to ground. The current transistors (Q) have the same characteristics as the reference transistor (QR), and each current transistor has a base terminal, a collector terminal and an emitter terminal. The base terminals of the current transistors (Q) are connected to the collector terminal of the reference transistor (QR). The collector terminals of the current transistors (Q) are connected respectively to the driving pins. The emitter terminals of the current transistors (Q) are connected to ground. All current transistors (Q) must operate in its active region so the currents on the driving pins will be the same, and the voltage on the collector terminal of each current transistor (Q) must be higher than an active voltage VC(ACT) of each current transistor (Q) so each current transistor (Q) will operate in its active region.
However, turn-on voltages of all LEDs (51) are not absolutely the same. Larger turn-on voltage of an LED (51) results in a lower voltage input to the collector terminal of the corresponding current transistor (Q). If any voltage input to the collector terminal of the current transistor (Q) is lower than the active voltage VC(ACT) of the current transistor (Q), the current transistor (Q) will not operate in its active region. Accordingly, the current on the driving pin corresponding to the current transistor (Q) not operating in its active region is not the same as the currents on the driving pins corresponding to the current transistors (Q) operating in their active regions. Different currents through the LEDs (51) result in different level of illumination of the LEDs (51).
To overcome the shortcomings, the present invention provides a light emitting diode circuit having even current to mitigate or obviate the aforementioned problems.